The present invention relates to a multi-layered catalyst on a metal substrate for the catalytic conversion of gases, such as the purification of exhaust gases of internal combustion engines. The present invention further relates to a method for producing a catalyst.
Catalysts for the purification of waste gases from an internal combustion engine or other industrial processes typically include a thermally resistant metal substrate, an alumina interlayer and a catalytically active layer containing platinum. The catalytically active materials of the interlayer and catalytically active layer purify exhaust gases such as CO, CH.sub.x and NO.sub.x. In known catalysts, delamination of the coating from the substrate is a common source of failure.
Stronger bonding of the alumina interlayer to the substrate is possible if an adhesive sublayer, which contains an element also found in the substrate, is deposited between the two layers. However, at the high service temperatures typically encountered in an internal combustion engine, the catalytically active layer is susceptible to failure at the sublayer/active layer interface.
Metal-ceramic interfaces are weak points in the composite. Dissimilar materials are sites for mechanical stress concentrations and often nucleate the failure process. Mechanical properties of the interface often control the overall function of the coated catalyst. Therefore, improving the mechanical properties of the interface can result in a prolonged life for the entire article.
Known catalysts still need a relatively large amount of noble metals to achieve sufficiently high conversions of CO, CH.sub.x and NO.sub.x. In 1989, Europe consumed 26 tons of platinum, most of which was used by the automobile industry in catalytic converters. Since metals of the platinum group are expensive and limited in resources, it is desirable to reduce the noble metal content of the catalyst without sacrificing catalytic efficiency.
It is the object of the present invention to provide a catalyst with a strong bond to the metal substrate which is stable to thermal and mechanical shocks. It is a further object of the present invention to provide a catalyst with improved catalytic activity while reducing the noble metal content in the active layer.
It is a further object of the present invention to provide a method of preparing a catalyst with improved thermal and mechanical stability and enhanced catalytic activity.